Often, it is important to know the tendency of solid matter present in a liquid containing solid matter to form deposits on surfaces. This can also be considered as the depositability of a substance, e.g. relative tendency of the solid matter in a substance to form deposits on surfaces. Additionally, this is sometimes discussed as tackiness or stickiness or depositability of a substance. This is particularly important within the paper production industry as well as certain other chemical manufacturing processes or systems containing liquids with solid matter, e.g. cooling waters. Particularly, it is often important to know or estimate depositability of the solid matter or solid contaminants present in a liquid, in particular in an aqueous substance. This is the case in a paper or board making processes. The depositability of the solid matter can be characterized, e.g. by the number of depositing particles on a surface during a measurement, size of depositing particles, number of depositing particles in certain size groups, an area of a measurement surface covered by depositing particles or any combination thereof. The depositing particles are often of hydrophobic nature.
Stickies are various adhesives which enter the paper making process with recycled fibers. These may be, for example; hot melts, pressure sensitive adhesives, envelope glues, contact glues, paper coating adhesives and/or printing ink binders. Stickies are formed or originate from recycled paper or board. Typical compositions of stickies are or include, for instance; acrylates, polyvinyl acetates and/or latexes or any polymeric material in recycled fiber. Additionally, wood pitch originating from mechanical or chemical pulp can cause tackiness. Stickies are tacky on paper machine fabrics, foils, cylinders and calendaring rolls. They can be responsible for significant production losses in the form of cleaning shut downs and web breaks. In other industrial water streams containing depositable, often hydrophobic, solid matter, may also cause problems in the processes.
A significant portion of the stickies in a substance can be removed mechanically in the recycled fiber processing in deinking plants or in OCC (old corrugated containers) plants. Any remaining stickies can be controlled by chemical means. For example, surfaces of sticky particles can be passivated with a chemical that makes them less tacky. Sticky particles can also be kept small in size with dispersing chemicals or they can be fixed to fiber surfaces with fixative or retention chemicals. It is therefore important to know how tacky a pulp is in order to control, manually and/or automatically, the processes and chemical additions for reducing or mitigating stickies, including selection of chemicals.
The depositing tendency of the stickies is often estimated by describing the tackiness of such substances. Existing methods for determining the tackiness of such substances or particulate material, which include plate tests and aeration tests, require samples to be taken from the production process.
There are several standardized macro stickies measurement methods: TAPPI method T277 (TAPPI 1999), INGEDE (International Association of the Deinking Industry) method no. 4, and a method by the International Organization for Standardization (ISO) based on screening and either visual inspection (ISO 15360-1:2000) or image analysis (ISO 15360-2:2001). These or slightly different methods are widely used in the industry and research.
Most laboratory methods that measure the amount of macro stickies are based on either handsheet making or screening of the pulp through 100 or 150 μm slots. These pre-treatments are followed by dyeing the stickies, the background, or both to have a greater contrast between the background and stickies. The stickies are then quantified as area (mm2/kg) or in number of stickies per kg of dry pulp. Though there may be a good correlation between the macro stickies analyzed by image analysis and screen rejects analyzed with DCM extraction and quantification of stickies with FTIR, macro stickies measurement do not always correlate with the deposit problem on paper machines.
Substantial limitations exist in current methods for determining depositing tendency on surfaces of a solid matter present in liquids. For instance, none of the existing methods allow for continuous determination of the depositability of the solid matter as it flows through a production process. Therefore, there exists a need for a method or device which is capable of determining depositability of the solid matter in a continuous manner, in particular on-line.
A further limitation of current methods for determining depositability of the particulate solid matter present in a liquid is that the process stream to be measured continuously or on-line, has to be fractionated before the measurement. That is, the particulate contaminants to be analyzed have to be separated from the sample before the measurement.
Another limitation of current methods for assessing depositability of a substance is the necessity to remove a sample from the production process. The process of removing a sample, or in the case of a plate test to remove a plate which has been exposed to the substance, and transporting the sample to a remote testing location uses both resources and time to determine tackiness. Not only is this generally inefficient but it can also mean that during the elapsed time between sampling and testing an undesired amount of flawed substance is allowed to pass through the sample site.
One or more of the above-mentioned limitations are seen in currently available measurement devices and methods for measuring depositability of particulate contaminants present in pulp and papermaking systems.
Different production processes and substances require differing degrees of accuracy and timeliness with regards to determining the tackiness or depositability of the substance. Therefore, there exists a need for a device or sensor capable of determining the depositability of a fluid substance quickly and without removing a measurement plate for remote analysis.